1. Technical Field
The present invention relates in general to reinforced pipes and, in particular, to an improved system, method, and apparatus for pre-tensioning riser pipes to share load with composite covers.
2. Description of the Related Art
In the prior art, the internal pressure capacity of steel alloy risers may be increased by wrapping the outer diameters (OD) of the risers with a composite material, such that the pressure capacity is shared by the steel pipe and the composite wrap. For example, in one application a pipe is wrapped with a carbon-fiber and epoxy matrix composite that is applied to the OD. The composite is strictly limited to increasing the hoop stress capacity of the pipe. In addition, a fiberglass covering can be applied over the top of both the composite and the pipe to provide damage protection.
During service, the pipe is subjected to high axial tension and bending moments, extremely high internal pressures, and environmental fatigue loads. Consequently, one manufacturing consideration is how to prevent the pipe from yielding prior to the internal pressure load being taken by the composite.
Several methods that address this issue have been suggested and include the following: (a) pre-yielding the composite-wrapped pipe by pressuring the inner diameter (ID) past the yield strength of the pipe to ensure load sharing (also known as “auto-frettage”); (b) letting the load sharing occur via deflection (with or without yielding) during normal service of the product; and (c) developing hoop stress by applying high tension to the carbon fibers during winding of the pipes (also known as “frettage”). Although these solutions are workable for some applications, improved solutions having even higher performance capabilities would be desirable.